“Desaturation” detection is commonly used in bipolar transistor and IGBT based power processors. A commonly used scheme is shown in FIG. 1. It consists of connecting diode to the collector of the device. The voltage on the anode of the diode is equal to its voltage drop plus the voltage drop across the power device. In case of an over current/short circuit condition, the voltage across the FET/IGBT rises and thus the anode voltage of the diode also rises. This voltage is fed (sometimes through a resistive divider) to a comparator with a preset limit. If the voltage goes above this limit, the device is turned off. This works well for IGBTs which have forward drops in the order of 2 to 3 volts and, as the diode D1's drop is 0.5V, it does not substantially add to sensed voltage.
One problem with this method is that when it is used in conjunction with low voltage MOSFETs which have low on-resistance (Rdson), the MOSFET voltage is about 0.18V and the diode drop is much higher than this voltage. As a result, the accuracy/tolerance of the trip current is very poor. It can vary over a wide range which is sometimes unacceptable in the application.